Eager load hosts for sites

* Get hosts data access out of the view
* Into a slightly-simplified SitesController with eager load for
  aka_hosts
Remove most_used_tags(_for_site)

* Get data access out of the helper
* Put it into Site#most_used_tags
* Optimise against our case rather than AATO's general case
  (AATO's case gives three subqueries, ours is simple JOINs)